Organometallic materials, especially those with a controlled spatial arrangement, are playing an ever-expanding role in the semiconductor industry. Owing to their electric and dielectric properties, they are employed in solar cells, light-emitting diodes, photodetectors, transistors and microchips. In order to produce technical functional units, it is indispensable in many cases to apply the organometallic material in the form of a thin, homogenous layer that covers the flat substrate. However, up until now, such functional units on the basis of organometallic materials could only be produced over very small crystalline regions.
The Langmuir-Blodgett technique may be employed to produce thin organic films that consist of monolayers up to a few hundred layers and that generally exhibit non-linear properties which are utilized, for instance, to generate frequency-doubled light. A basic prerequisite in this context is the anisotropic orientation of the molecules. Here, as well, in addition to purely organic molecules, organometallic compounds are now playing an increasing role.
Up until now, organometallic layers have usually been produced by means of deposition processes and they display crystalline regions with numerous crystal boundaries.
F. Ghosh, W. M. Lackowski and R. M. Crooks, “Two New Approaches for Patterning Polymer Films Using Templates Prepared by Microcontact Printing”, Macromolecules, volume 34, pages 1230 to 1236, and X. Chen., L. M. Tolbert, C. L. Henderson, D. W. Hess, and J. Rühe, “Polymer pattern formation on SiO2 surfaces using surface monolayer initiated polymerization”, J. Vac. Sci. Technol., volume 19(6), pages 2013 to 2019, 2001, describes the lateral structuring and activation of substrates and subsequent polymerization of three-dimensional macromolecular objects. With this approach, however, so far it has only been possible to produce laterally structured organic layers that are a few nanometers thick, in addition to which the steepness of the edges of the structures is insufficient.
S. D. Evans, A. Ulman, K. E. Goppert-Berarducci, and L. J. Gerenser, “Self-Assembled Multilayers of ω-Mercaptoalkanoic Acids: Selective Ionic Interactions”, J. Am. Chem. Soc., volume 113, pages 5866 to 5868, 1991, and S. D. Evans, T. M. Flynn, and A. Ulman, “Self-Assembled Multilayer Formation on Predefined Templates”, Langmuir, volume 11, pages 3811 to 3814, 1995, describes a method in which, at first, a passivating monolayer of a first molecule is stamped and subsequently, a monolayer of a second molecule, which has a polar terminal group (carboxyl), is inserted into the interstices. Following a thorough washing procedure, the specimen is exposed to a copper acetate solution, a process in which a thin copper ion layer is formed on the top surface of the self-organizing monolayer. After another washing procedure, the specimen is once again exposed to the second type of molecule, a process in which another monolayer is created. The layer can be slowly built up by repeating the process steps. Film thicknesses of up to 50 nm have been achieved employing this method. A drawback of this is that the continuous changing of the process baths and the long retention times of the specimen in these baths render this process very complex and time-consuming.
This method is closely related to approaches employed by R. Maoz and J. Sagiv, “Targeted Self-replication of Silane Multilayers”, Adv. Mater., volume 10, no. 8, pages 580 to 584, 1998, with which, however, it was only possible to obtain layers having a thickness of just a few nanometers. By means of force microscopy in S. Liu, R. Maoz, G. Schmid and J. Sagiv, “Template Guided Self-Assembly of [Au55] Clusters on Nanolithographically Defined Monolayer Patterns”, Nanoletters, volume 2, no. 10, pages 1055 to 1060, 2002, very fine lines were activated for the growth and decorated with larger metal objects (gold clusters).
W. Li, V. Lynch, H. Thompson and M. A. Fox, “Self-Assembled Monolayers of 7-(10-thiodecoxy)coumarin on Gold: Synthesis, Characterization, and Photodimerization”, J. Am. Chem. Soc., volume 119, pages 7211 to 7217, 1997, describes that many thiol molecules, including coumarin thiols, form a densely-packed, self-organizing monolayer on the surface of coinage metals.
B. H. Hong, S. C. Bae, C.-W. Lee, S. Jeong and K. S. Kim, “Ultrathin Single-Crystalline Silver Nanowire Arrays Formed in an Ambient Solution Phase”, Science, volume 294, pages 348 to 351, 2001, describes silver nanowires that are formed in a catalytic, crystalline, organic host material under ambient conditions. Monocrystalline regions that are filled with these silver nanowires were produced and examined by means of X-ray diffraction.
I. G. Dance, K. J. Fisher, R. M. H. Banda and M. L. Scudder, “Layered Structure of Crystalline Compounds AgSR”, Inorg. Chem., volume 30, pages 183 to 187, 1991, describes a crystalline powder from the precipitation reaction of the AgSR type, wherein R stands for an organic radical that consists essentially of a central plane made up of silver atoms in a quasi-hexagonal arrangement that are bonded by bridging SR groups whose organic part faces in both directions parallel to the surface normal and thus perpendicular to the silver planes.
A. N. Parikh, “Characterization of Chain Molecular Assemblies in Long-Chain, Layered Silver Thiolates: A Joint Infrared Spectroscopy and X-Ray Diffraction Study”, J. Physical Chemistry B, volume 103, pages 2850 to 2861, 1999, reports on examinations carried out on these crystalline thiolate materials (all powders from a precipitation reaction) and shows that the thiol molecules arrange themselves head-to-head through the mediation of van der Waals interaction. X-ray examinations of these substances reveal an entire set of periodical reflections, as is characteristic of a well-ordered plate system. The distance of the plates thus obtained corresponds exactly to twice the molecule length. The sulfur-to-silver ratio is 1:1.
H. G. Fijolek, P. Gonzalez-Duarte, S. H. Park, S. L. Suib and M. J. Natan, “Structure-spectroscopy correlations in silver thiolates: Application to the structure of silver 1,5-pentanedithiolate”, Inorg. Chemistry, volume 36, pages 5299 to 5305, 1997, and H. J. Choi, S. W. Han, S. J. Lee and K. Kim, “Temperature-dependent FT-IR spectroscopy study of silver 1,9-nonanedithiolate”, Applied Spectroscopy, volume 55, pages 1085 to 1091, 2001, describe dithiolates that are likewise produced by precipitation reactions. Here, too, structures with a lamellar arrangement are created. However, X-ray examinations reveal much wider reflections, which are an indication of a less ordered structure.